Hollow Fiber Delivers Distortion-Free Femtosecond Pulses

Multiphoton fluorescence is an important technique for analyzing living tissue, and fiber optic delivery of the femtosecond pulses enables useful instruments such as miniature microscopes and multiphoton endoscopes. A difficulty generally arises, however, because the peak power in the ultrashort pulses exceeds the threshold for nonlinear effects in glass fiber at very low pulse energies. Recently, a research group at Max Planck Institut für Medizinische Forschung in Heidelberg, Germany, demonstrated the delivery of high-energy femtosecond pulses through a commercial, hollow-core photonic bandgap fiber with no nonlinear effects.

Unlike conventional photonic crystal fibers, in which light is guided by total internal reflection because the average refractive index of the cladding is less than that of the core, photonic bandgap fibers guide light because the periodicity of the cladding prohibits light propagation in the cladding. The refractive index of the core is irrelevant to the wave-guiding mechanism. Thus, single-mode, hollow-core fibers have been fabricated and recently entered the commercial market. Because light propagates in the hollow (air-filled) core, nonlinear effects have a much higher threshold than in conventional glass fibers.

Figure 1. There was virtually no broadening of the pulse that passed through the hollow photonic bandgap fiber at its zero-dispersion wavelength, but there was significant broadening at other wavelengths. The spectral width was unchanged across the tuning range. The circles are data taken at an average power of 2 mW, and the triangles are data taken at 250 mW from the 76-MHz laser.Dispersion is another problem that plagues the delivery of femtosecond pulses through glass fibers. Because the zero-dispersion wavelength for most glass fibers is beyond the spectral range of Ti:sapphire lasers, the pulses are broadened by dispersion as they pass through the fiber. Although dispersion can be compensated by chirping the pulse before it enters the fiber, this adds an undesirable level of complexity. In a hollow fiber, however, the dispersion of air is negligible in comparison with the waveguide dispersion, which passes through zero within the spectral range of Ti:sapphire lasers at approximately 812 nm. So a Ti:sapphire laser pulse can propagate in a hollow-core fiber with minimal dispersion broadening, even when it is not chirped in advance.

The researchers used a 1.5-m length of HC-800-01 commercial photonic bandgap fiber from BlazePhotonics Ltd. in Bath, UK, that had a 9-µm-diameter core surrounded by a 40-µm-diameter, microstructured cladding. The fiber transmitted more than 50 percent of the incident energy and maintained the Gaussian intensity profile of the input beam.

Figure 2. When the laser was tuned to the fiber's zero-dispersion wavelength, the spectral width of the pulse passing through the fiber was independent of power, and the pulse width was only slightly dependent on power.When they tuned their Ti:sapphire laser from 790 to 850 nm, they observed pulse broadening on either side but virtually none at the zero-dispersion wavelength (Figure 1). The wavelength dependence of the broadening was independent of power, indicating that the effect is due to dispersion and has no component caused by nonlinear effects. The spectral width also was unchanged across the tuning range.

With their laser tuned to the fiber's zero-dispersion wavelength of 812 nm, the researchers observed that the spectral width of pulses transmitted through the fiber was unchanged as the average power of the 76-MHz laser was increased from 0 to 350 mW (Figure 2). The pulse duration increased only slightly, roughly 15 percent, over the same power range.